Hyaluronan (HA) is a major component of the extracellular matrix. It plays an important role in the mechanical functions of the extracellular matrix and stabilization of cells. Currently, its mechanical properties have been investigated only at the gross level. In this study, the mechanical properties of single HA molecules were directly measured with an optical tweezer technique, yielding a persistence length of 4.5±1.2nm. This information may help us to understand the mechanical roles in the extracellular matrix infrastructure, cell attachment, and to design tissue engineering and drug delivery systems where the mechanical functions of HA are essential.